Geology Reference
In-Depth Information
(a)
(b)
300 m
300 m
2003
2005
Fig. 3.
High-resolution remote sensing image of pre- (a) and post- (b) storm morphology of a portion of the barrier reef and
its reef apron. This fi gure demonstrates the absence of signifi cant geomorphological change (e.g. no expanded reef apron)
during a period that included the passage of hurricanes Frances and Jeanne in 2004. Although there are some expanded
sand patches around some patch reefs, no major geomorphological changes occur. (The whitish colour areas on the
pre-storm image are waves breaking on the reef crest, and the darker patches in the apron on the pre-storm image are
most probably due to seasonal differences in seagrass colonization rather than depth differences.) Image courtesy of
DigitalGlobe.
was either eroded from or deposited to the
existing system through the available datasets.
seagrass-stabilized area on the shoal (black dashed
box in Fig. 4c). In the 2005 image, this small region
has been 'fi lled in' with sand. The crest of the
northern lobe in the area of the black dashed box
of Fig. 4c migrated ~30 m westward, whereas the
adjacent crest southwest of this one did not move
to the same extent. The linear bar (Fig. 1) perpen-
dicular to this region also broadened.
The fl ood deltas showed the most obvious
changes over this 2-year period. The sandy extent
of some deltas expanded ~30-60 m out onto
the platform and ~10-20 m laterally (relative
to the central channel). Many of the sand wave
crests of the fl ood deltas followed similar trends to
those on the ebb deltas and migrated to the west-
southwest. Some of the crests, however, move
islandward (solid black box in Fig. 4c), and others
move to the east.
These observations indicate that the storms did
modify the carbonate platform, at least to some
extent. The crests of some sand waves migrated
30+ m. Such modifi cations to the shoals may be
due to the passage of the two storms. With the
direction of approach of both Hurricane Frances
and Hurricane Jeanne, the strongest winds would
have come from the northeast, which may account
for the southwestern migration patterns. However,
storms should not be considered the only possible
explanation for these morphological adjustments.
What changes resulted from the passage of the
two hurricanes?
Comparisons between these two datasets
demonstrate very little geomorphological change
in the study area. In many areas, the two images
appear more-or-less identical (Fig. 3), illustrating
little to no change. For example, the reef aprons
show minimal morphological change after the
passage of the two storms, and the majority of
the shelf behind the reef does not illustrate any
noticeable change in the remote sensing images.
There are, however, some minor modifi cations to
the platform after the storm passage. In the shelf
region behind the reef, just seaward of the deltas,
two of the linear bars and blowouts (Fig. 1) expe-
rienced some seagrass removal and slight (<10 m)
eastward expansion of the blowouts.
The most pronounced changes in the study area
occur in the crests of the tidal deltas (Fig. 4) where
some sand wave crests migrated ~30-35 m, but
none of the crests moved more than 50 m. On the
ebb lobes, some shoal crests migrated to the south-
east. The largest change in the ebb lobes occurred
in the southwestern portion of the area illustrated
in Fig. 4. In the 2003 image, there is a small (~35 m)
